1. Field of the Invention
This invention relates generally an artificial lumbar disc made of three components. The prosthesis is inserted into a disc cavity to restore natural function of the disc.
2. Description of the Prior Art
Back pain is the leading cause of disability and worker compensation claims in the US for people under 45 years of age. The cost of treating back pain varies according to different studies but numbers as high as $26 billion in direct cost and $90 billion in total cost are frequently cited.
The normal human disc is comprised of an outer fibrous structure (annulus fibrosus) with fibrous bands organized like the plies of a radial tire. The annulus fibrosus controls the motion of the vertebral segment. The inner core or nucleus pulposus has a water content of 70 to 85%. It transmits and dampens axial loading (“shock absorbing” function). In degenerative disc disease, the annulus develops structural lesions that weaken its ability to control segmental movements and to contain the central core. Also, the nucleus loses part of its water content. The combined results provide for segmental instability and a decrease in the shock absorbing function of the disc. Spinal stenosis, osteophyte formation, disc herniation, and possible nerve root compression are associated with disabling back pain.
At the present time, when conservative measures fail to provide relief for the patient, the most common available surgical procedures consist of discectomy (in variable amount of completeness) or arthrodesis (spinal fuision) of the involved segment. Though initially successful in relieving pain in 50 to 90% of cases, many discectomy patients experience return of pain and may require additional surgery, often in the form of a spinal fusion. Arthrodesis techniques provide for stability of the affected segment and solid fusions are now reported in the 90 to 95% range though pain relief is present in only 75 to 80% of cases depending on the series reported. Unfortunately, as longer follow-up studies become available, accelerated degeneration of the unfused segments above and below the arthrodesis presents an increasing problem.
Spinal surgeons have become progressively more aware of this phenomenon, opening the door for new techniques that can preserve the motion of the vertebral segment with artificial disc prostheses. This has resulted in a more physiological approach to intervertebral disc disease. Artificial disc prostheses are now in different stages of development. At the present time, only one artificial disc has been approved for patient use in the US. It has a ball and socket design, requires a major invasive surgical procedure, and does not address the issue of the “shock absorbing” function of the normal human intervertebral disc. Other prostheses now at different stages of development are also of the ball and socket design and/or require major invasive surgery for adequate implantation.